Add Info, that "conceal" is needed

[tjwelde]

I wondered, why conceal didn't work for me. After looking through the code of this plugin, I found out, that you have to have vim compiled with the conceal feature enabled. I didn't know of the conceal plugin before and just have been using the default MacOS vim. To prevent others to stumble into this problem, it would be nice to present this information in some form. Best Regards